This invention relates to a method of shutting off a portion of a producing zone in a producing wellbore through the use of a concrete plug.
The methods of plugging wellbores are numerous. Many procedures have been around for decades. Some involve nothing more than filling a hole with concrete, or as in the case of U.S. Pat. No. 3,831,383, using a prefabricated plug upon which a plastic or cement is poured.
A more difficult task than merely plugging a wellbore is trying to close off a portion of a producing zone while still leaving part of the producing interval open for production. In a steamflood for instance, a steam override zone may develop in the top portion of a producing interval, substantially inhibiting recovery of hydrocarbons from a producing well. One solution to the steam override problem is to drill a new well if the producing zone is within a couple of hundred feet of the surface.
A second solution that is employed commercially is to run a scab liner. This procedure involves hanging several hundred to several thousand feet of pipe inside the existing wellbore to cover the steam override zone, cementing around the steam override zone, and perforating at the bottom of the producing interval. But this solution suffers from the problem of high cost for many feet of extra steel and has the additional complexity of hanging the liner in the hole. A scab liner job also decreases the inside diameter of the wellbore. A smaller wellbore decreases the efficiency of workovers that can be performed upon that well.
For example, production wells used in steamfloods are subject to frequent workovers. The old liner has to be pulled and the damaged hole must be scraped away to get to the fresh native reservoir rock. This is done with the use of an underreamer which spreads out arms an effective working diameter larger than the casing to scrape the damaged hole clean in the production interval. The smaller the hole, the smaller sized underreamer that can be used. With a smaller underreamer, the chances of torquing off an arm of the underreamer or getting the tool stuck in the hole are substantially increased. Thus, running a smaller diameter liner in the hole to shut off a steam override zone reduces the number of future workovers that can be performed and increases the problems associated with such workovers.
Isolating the undesired portion of the producing zone with packers is generally unsatisfactory. Steam from an override zone will flow through the gravel pack or adjacent formation behind the liner and bypass the packers.
Another procedure that has been explored for shutting off a portion of a producing zone has been aptly named a cement donut. In this method, a cement plug is formed in the wellbore in the portion of the producing interval that is desired to be shut off. The operator drills through the cement plug in an attempt to make a new wellbore through the cement plug, while leaving the resulting cement donut to shut off the relevant portion of the producing interval. The cement donut technique, however, has not been successful due to the tendency of the drill bit to drift through the side of the cement sheath, exposing the portion of the producing interval to the wellbore.